1. Field of the Invention
The present invention relates to the field of printed circuit boards. More specifically, the present invention relates to using a surface mountable grounding spring to ground a printed circuit to an enclosure sub-assembly or chassis which is referenced to ground and contains the circuit board.
2. The Background Art
A printed circuit board (PCB) generally emits electromagnetic interference (EMI). Electromagnetic compatibility (EMC) standards strictly define 1) allowable levels of EMI which may be emitted by an electronic device and 2) levels of EMI to which an electronic device must be immune. In order to meet these EMC standards, it is generally necessary to tie the ground of a printed circuit board to the grounded electrically conductive enclosure sub-assembly or chassis containing the circuit board.
Traditionally, this grounding has been accomplished using a screw and standoff. FIG. 1 is a diagram illustrating a PCB grounded to an enclosure using a screw and standoff. Standoff 10 may be attached to an enclosure sub-assembly 12 by press fitting or using some other attachment technique. PCB 14 is then attached to the standoff 10 using a screw 16. The standoff 10 has a hole in its interior with threads for receiving the screw 16. Screw 16 is passed through a hole in the PCB and threaded into the hole in the standoff 10, securing the PCB to the standoff. The standoff then acts as a conductor to ground the PCB to the enclosure sub-assembly.
FIG. 2 is a perspective view of the standoff 10 of FIG. 1. A hole 20 in the standoff 10 leads to an open shaft 22 containing threads for receiving a screw. The standoff 10 is generally cylindrical in shape, but there are many different shapes which may be used for the standoff 10.
There are several drawbacks to using the screw and standoff method of grounding a PCB to an enclosure sub-assembly. First, a hole must be placed in the PCB for the screw to pass through and attach to the standoff. However, the space taken up by even a small hole in the printed circuit board wastes valuable portions of the PCB. Without the hole, the PCB could contain more routing connections or other circuits. Designers of the printed circuit board must design around a hole in the circuit board, adding time and money to the design phase of the board. In many situations designers cannot ground the PCB to the chassis where needed because nearby components prevent placing a hole in the PCB.
Second, there is a great deal of assembly time and labor cost added due to the manual operation of inserting the screw through the PCB and into the standoff. It would be preferable to design a grounding apparatus that could be assembled by a machine.
Third, the cost of both a standoff and a screw can be expensive since there are two separate parts that must be purchased or manufactured. It would be preferable to accomplish the same tasks using a single piece of hardware.
An alternative design for grounding a PCB to an enclosure sub-assembly was created by Boldt Metronics International (BMI). This design utilizes a spring to ground the PCB to the enclosure sub-assembly. FIG. 3 is a diagram illustrating a perspective view of the BMI spring. It comprises a substantially flat base portion 40, a substantially flat inclined portion 42, a substantially flat head portion 44, a substantially flat first flap 46a, and a substantially flat second flap 46b. One end of the inclined portion 42 is adjacent to the base portion 40 and the other end of the inclined portion 42 is adjacent to the head portion 44. Each of the first flap 46a and the second flap 46b have one side adjacent to the head portion 44 as shown. The junction between the head portion 44 and the inclined portion 42 is formed by a bend which produces tension in the spring. Likewise, the junction between the base portion 40 and the inclined portion 42 is formed by a bend which also produces tension in the spring. A hole 48 is sometimes provided at the junction between the base portion 40 and the inclined portion 42 to alter the tension in the spring. Likewise, another hole 50 is provided at the junction between the head portion 44 and the inclined portion 42 for the same purpose.
FIG. 4 is a diagram illustrating a front view of the BMI spring. FIG. 5 is a diagram illustrating a side view of the BMI spring. FIG. 6 is a diagram illustrating a bottom view of the BMI spring.
The base portion 40 is soldered to a PCB. Because there is no screw mechanism, a hole need not be made in the PCB so there is significantly more room on the PCB for additional circuity. The enclosure sub-assembly is then placed against the PCB such that the head portion abuts the enclosure sub-assembly, allowing grounding to occur. The flexibility of the spring allows for more room for maneuverability in the distance between the PCB and the enclosure sub-assembly. Thus, small variations in the distance between the two will not cause damage to the PCB and the connection between the two will be maintained.
The BMI spring, however, has several drawbacks. First, it is still necessary to manufacture different sized springs for products whose distances between the PCB and the enclosure sub-assembly vary greatly. Each of the springs is a small component that must be shaped and formed from metal which requires separate and unique tooling. Thus, producing additional designs adds a significant cost to the price of manufacture. It would be advantageous if a single spring design could be produced that would be compatible with a variety of products.
Second, during installation of the PCB onto the chassis, there is a tendency for the head portion of the spring to get snagged on nearby objects. This increases the time required to install the PCB and can even lead to damage of the spring or nearby components.
Third, the bond between the base portion of the BMI spring and the PCB is not always strong enough. The screw and standoff method benefitted from the fact that if a force was applied to move the standoff upwards, a reciprocal downwards pushing force would be applied by the junctions between the threads of the screw and the standoff. The BMI spring only has a flat portion soldered to another flat portion, thus a peeling force on the spring does not encounter sufficient sheer strength, but instead primarily its tensile strength. It would be preferable to have a design that could have more sheer strength in the joint between the spring and the solder.
Fourth, the BMI spring is formed by cutting a flat sheet of metal and then bending it in the appropriate directions. When machines bend metal, however, there is a tendency for the metal to tear at the bending point, especially when bending to a rigid 90 degree angle. It would be preferable to have a design that was less prone to tearing.